The present invention relates to a matrix type piezoelectric/electrostrictive device. More particularly, the present invention relates to a matrix type piezoelectric/electrostrictive device which is used for an optical modulator, optical switch, electrical switch, microrelay, microvalve, transportation device, image display device such as a display and a projector, image drawing device, micropump, droplet discharge device, micromixer, microstirrer, microreactor, various types of sensors, and the like, generates large force and large displacement, allows a piezoelectric/electro-strictive substance to generate expansion/contraction displacement or expansion/contraction vibration in a direction perpendicular to a main surface of a ceramic substrate by a transverse effect of an electric field induced strain of the piezoelectric/electrostrictive substance, and applies action such as pushing, distorting, moving, striking (impacting), or mixing to an object of action or operates when such action is applied, and to a method of manufacturing the matrix type piezoelectric/electrostrictive device.
In recent years, displacement control elements capable of adjusting the length or position of an optical path on the order of sub-microns have been demanded in the field of optics, precision machining, manufacture of semiconductors, and the like. To deal with this demand, development of piezoelectric/electrostrictive devices such as actuators or sensors which utilize strain based on a reverse piezoelectric effect or an electrostrictive effect occuring when an electric field is applied to ferroelectrics or antiferroelectrics has progressed. The displacement control elements utilizing an electric field induced strain are capable of easily controlling minute displacement, decreasing power consumption due to high mechanical/electrical energy conversion efficiency, and contributing to a decrease in the size and weight of a product due to ultraprecise mounting capability in comparison with a conventional electromagnetic method or the like using a servomotor or a pulsemotor. Therefore, application fields of displacement control elements are expected to be expanded steadily in the future.
Taking an optical switch as an example, use of a piezoelectric/electrostrictive device as an actuator for switching a transmission path of introduced light has been proposed. FIGS. 2(a) and 2(b) show an example of an optical switch. An optical switch 200 shown in FIGS. 2(a) and 2(b) includes alight transmitting section 201, an optical path change section 208, and an actuator section 211. The light transmitting section 201 includes a light reflecting surface 101 provided on part of a surface which faces the optical path change section 208, and light transmitting paths 202, 204, and 205 provided in three directions from the light reflecting surface 110. The optical path change section 208 includes a light introducing member 209 which is moveably provided close to the light reflecting surface 101 in the light transmitting section 201 and formed of a light transmitting material, and a light reflecting member 210 which totally reflects light. The actuator section 211 includes a mechanism which is displaced by an external signal and transmits the displacement to the optical path change section 208.
As shown in FIG. 2(a), the actuator section 211 operates (displaces) by applying an external signal such as a voltage. The optical path change section 208 is separated from the light transmitting section 201 by the displacement of the actuator section 211. Light 221 introduced into the light transmitting path 202 in the light transmitting section 201 is totally reflected by the light reflecting surface 101 in the light transmitting section 201, of which the refractive index is adjusted at a specific value. The reflected light 221 is transmitted to the light transmitting path 204 on the output side.
As shown in FIG. 2(b), the actuator section 211 returns to the original position when the actuator section 211 enters a non-operating state, whereby the light introducing member 209 in the optical path change section 208 comes in contact with the light transmitting section 201 at a distance equal to or less than the wavelength of the light. As a result, the light 221 introduced into the light transmitting path 202 is introduced into the light introducing member 209 from the light transmitting section 201 and transmitted through the light introducing member 209. The light 221 transmitted through the light introducing member 209 reaches the light reflecting member 210. The light 221 is reflected by the light reflecting surface 102 of the light reflecting member 210 and transmitted to the light transmitting path 205, differing from the light reflected by the light reflecting surface 101 in the light transmitting section 201.
As described above, the piezoelectric/electro-strictive device is suitably used as the actuator section of the optical switch having a function of changing the optical path. In particular, in the case of forming a matrix switch which switches between a plurality of channels, a piezo-electric/electrostrictive device in which a plurality of uni-morph or bi-morph piezoelectric/electrostrictive elements (hereinafter may be referred to as xe2x80x9cbending displacement elementsxe2x80x9d) are arranged, as disclosed in Japanese Patent No. 2693291 issued to the applicant of the present invention, is suitably employed. The bending displacement element consists of a diaphragm and a piezoelectric/electrostrictive element, and generates bending displacement by converting only a small amount of expansion/contraction strain of the piezoelectric/electrostrictive element produced when applying an electric field into a bending mode. Therefore, a large displacement is easily obtained in proportion to the length of the piezo-electric/electrostrictive element.
However, since the bending displacement element converts strain, stress which causes the strain of the piezoelectric/electrostrictive element cannot be directly utilized. Therefore, it is very difficult to increase displacement and force generation at the same time. Moreover, since the resonance frequency is inevitably decreased as the length of the element is increased, it is also difficult to satisfy a desired response speed.
In order to further improve the performance of the above type of optical switch, there have been at least the following two demands. Specifically, an increase in ON/OFF ratio (contrast) is demanded. In the case of the optical switch 200, it is important to securely perform contact/separation operations between the light transmitting section 201 and the optical path change section 208. Therefore, the actuator section preferably has a large stroke, specifically, generates large displacement.
The other demand is to minimize a loss caused by switching. In this case, it is important to increase a substantial contact area between the optical path change section 208 and the light transmitting section 201 while increasing the area of the optical path change section 208. However, since an increase in the contact area causes reliability relating to separation to be decreased, a piezoelectric/electrostrictive device capable of generating a large force is necessary as the actuator section. Specifically, a piezoelectric/electrostrictive device capable of generating displacement and force at the same time is demanded as the actuator section in order to improve the performance of the optical switch.
It is preferable that each of the piezoelectric/electrostrictive elements be formed independently. This means that each of the piezoelectric/electrostrictive elements does not interfere with the others, specifically, does not restrict displacement and force generated by other piezoelectric/electrostrictive elements.
For example, a piezoelectric/electrostrictive device 145 shown in FIG. 3 shows bending displacement by the operation of piezoelectric/electrostrictive elements 178, as shown in the cross section in FIG. 4. Each of the piezoelectric/electrostrictive elements 178 is formed to be mechanically independent from the adjacent piezoelectric/electrostrictive elements by utilizing the rigidity of a partition 143.
However, a substrate 144 has an integral structure, and a vibration plate, on which the piezoelectric/electro-strictive element 178 acts, is continuously formed. Therefore, it cannot be denied that tension or compressive stress of the vibration plate which occurs by the operation of the piezoelectric/electrostrictive element 178 affects the adjacent piezoelectric/electrostrictive elements, although the adjacent piezoelectric/electrostrictive elements are separated by the partition 143. This particularly applies to a case where the piezoelectric/electrostrictive elements are formed at a high density.
In a piezoelectric/electrostrictive device 155 shown in FIG. 5 (cross-section), since sidewalls 219 which support a vibration plate 218 are separated from the adjacent side walls 219, the adjacent piezoelectric/electrostrictive elements are not affected.
As another embodiment of the piezoelectric/electrostrictive device in which each of the piezo-electric/electrostrictive elements is formed independently, Japanese Patent No. 3058143 proposes an actuator in FIG. 1. This actuator is a piezoelectric actuator suitable for a ink-jet type recording device, in which pillar-shaped piezoelectric elements which function as drive mechanisms are arranged in rows and columns. Japanese Patent No. 3058143 states that a plurality of piezoelectric elements can be highly integrated on a substrate in rows and columns by employing piezoelectric transverse effect type piezoelectric elements having a simple electrode configuration, and the number of ink-jet nozzles per unit area in the ink-jet type recording device can be increased.
The piezoelectric actuator disclosed in this patent is formed by stacking and sintering green sheets to which common electrodes or signal applying electrodes are applied, and by forming grooves using a dicing saw so that the pillar-shaped piezoelectric elements are separated. Therefore, this piezoelectric actuator has at least the following two problems.
Since this piezoelectric actuator has a structure in which driver electrodes are stored in the piezoelectric element in advance, an electrode-piezoelectric material stacked structure of each of the piezoelectric elements becomes non-uniform due to the influence of strain during sintering. This causes characteristics of the elements to become uneven. Since the size (width or thickness) must be increased taking strain during sintering into consideration, it is difficult to decrease the pitch of the elements. According to the configuration example disclosed in Japanese Patent No. 3058143, since the width of the piezoelectric element is 0.3 mm and the width of the groove is 0.209 to 0.718 mm, one piezoelectric element is disposed per mm2. Such a degree of integration is insufficient to deal with the resolution required for ink-jet printers in recent years.
Moreover, the above degree of integration is insufficient for optical switches represented by the embodiment shown in FIGS. 2(a) and 2(b). The number of channels of optical switching devices is expected to be increased as construction of an optical network system without photoelectric conversion progresses. Therefore, a decrease in the size of optical switching devices and an increase in the degree of integration of optical switches used for the optical switching devices will be demanded in order to reduce transmission loss of signals. However, the degree of integration of the above piezoelectric actuator cannot deal with such a demand.
Each of the piezoelectric elements of the piezoelectric actuator disclosed in Japanese Patent No. 3058143 is formed by dicing saw processing. However, the depth of the grooves, specifically, the height of the piezoelectric elements is limited due to limitations relating to the processing. Since displacement of the transverse effect type elements depends on the height of the piezoelectric element, sufficient displacement cannot be obtained if the height of the element is limited. Specifically, the aspect ratio (height/thickness) of the piezoelectric element (piezoelectric), which is an index of the degree of integration and characteristics, cannot be increased. Therefore, the above piezoelectric actuator is not suitable as the actuator section for not only ink-jet printers, but also optical switches and the like.
As described above, a piezoelectric/electro-strictive device such as an actuator which is capable of generating displacement and force and in which piezoelectric/electrostrictive elements can be disposed independently at an extremely high density is demanded. The present invention has been achieved to deal with this demand. Specifically, an object of the present invention achieved in view of the above situation is to provide a piezoelectric/electrostrictive device which generates large displacement at a low voltage, has a high response speed, generates a large force, excels in mounting capability, enables a higher degree of integration, and applies action such as pushing, distorting, moving, striking (impacting), or mixing to an object of action, or operates when such action is applied, and a method of manufacturing the piezoelectric/electrostrictive device.
The present invention aims at improving the performance of an optical modulator, optical switch, electrical switch, microrelay, microvalve, transportation device, image display device such as a display and a projector, image drawing device, micropump, droplet discharge device, micromixer, microstirrer, microreactor, various types of sensors, or the like by applying the piezoelectric/electrostrictive device thereto. As a result of extensive studies, the present inventors have found that the above object can be achieved by the following matrix type piezoelectric/electrostrictive device and a manufacturing method thereof.
The present invention provides a matrix type piezoelectric/electrostrictive device in which a plurality of piezoelectric/electrostrictive elements almost in the shape of a pillar, each having a piezoelectric/electrostrictive substance and at least a pair of electrodes, are vertically provided on a thick ceramic substrate, and which is driven by displacement of the piezoelectric/electrostrictive substance. This matrix type piezoelectric/electrostrictive device is characterized in that a plurality of the piezoelectric/electrostrictive elements are jointed integrally to the ceramic substrate and are independently arranged in two dimensions, the pair of electrodes is formed on the sides of the piezoelectric/electrostrictive substance, the crystal grains on at least the sides of the piezoelectric/electrostrictive substance on which the electrodes are formed is in such a state that the percentage of transgranularly fractured crystal grains is 10% or less, and a curved surface is formed at the vicinity of a joining section between the piezoelectric/electrostrictive substance and the ceramic substrate.
In the matrix type piezoelectric/electrostrictive device according to the present invention, the degree of surface profile of the piezoelectric/electrostrictive substance of the piezoelectric/electrostrictive element is preferably about 8 xcexcm or less. The ratio of the height of the piezoelectric/electrostrictive element almost in the shape of a pillar to the shortest distance through the center axis in the horizontal cross section of the piezoelectric/electrostrictive element (hereinafter may be called xe2x80x9cthe thickness of the piezoelectric/electrostrictive elementxe2x80x9d) is preferably about 20:1 to 200:1. The shortest distance through the center axis in the horizontal cross section of the piezoelectric/electrostrictive element is preferably 300 xcexcm or less.
In the matrix type piezoelectric/electrostrictive device according to the present invention, the ratio of the height of the piezoelectric/electrostrictive element almost in the shape of a pillar to an interval between the adjacent piezoelectric/electrostrictive elements is preferably about 20:1 to 200:1. The sides of the piezoelectric/electro-strictive substance preferably have an almost uniform surface state, and surface roughness represented by Rt of the sides of the piezoelectric/electrostrictive substance is preferably 9 xcexcm or less, and surface roughness represented by Ra of the sides of the piezoelectric/electrostrictive substance is preferably 0.1 to 0.5 xcexcm. The radius of curvature of the curved surface formed near the joined section between the piezoelectric/electrostrictive substance and the ceramic substrate is preferably 20 to 100 xcexcm.
In the matrix type piezoelectric/electrostrictive device according to the present invention, the horizontal cross section of the piezoelectric/electrostrictive substance of the piezoelectric/electrostrictive element almost in the shape of a pillar is preferably in the shape of a parallelogram, and the electrodes are preferably formed on the sides including the long sides of the cross section of the piezoelectric/electro-strictive substance. The matrix type piezoelectric/electrostrictive device according to the present invention is a piezoelectric/electrostrictive device capable of utilizing either a longitudinal effect or a transverse effect of an electric field induced strain of the piezoelectric/electrostrictive substance. The piezoelectric/electro-strictive element is preferably expanded/contracted in a direction vertical to a main surface of the ceramic substrate based on displacement caused by the transverse effect.
As materials for the matrix type piezoelectric/electrostrictive device according to the present invention, the ceramic substrate and the piezoelectric/electrostrictive substance which makes up the piezoelectric/electrostrictive element are preferably formed of the same material. As the material for the piezoelectric/electrostrictive substance, any of piezoelectric ceramics, electrostrictive ceramics, and antiferroelectric ceramics, or a composite material of these ceramics and a polymer piezoelectric material may be suitably used.
In the above matrix type piezoelectric/electrostrictive device, wall sections may be formed between the adjacent piezoelectric/electrostrictive elements. Electrode terminals are preferably formed on the side of the ceramic substrate opposite to the side on which the piezoelectric/electrostrictive elements are disposed. The electrodes and the electrode terminals are preferably connected via through holes or via holes formed in the ceramic substrate.
The present invention also provides the following first and second methods of manufacturing a matrix type piezoelectric/electrostrictive device.
A first method of manufacturing a matrix type piezoelectric/electrostrictive device according to the present invention is a method of manufacturing a matrix type piezoelectric/electrostrictive device in which a plurality of piezoelectric/electrostrictive elements almost in the shape of a pillar are two-dimensionally arranged on a thick ceramic substrate, wherein each of the piezoelectric/electrostrictive elements includes a piezoelectric/electrostrictive substance and at least a pair of electrodes, the percentage of transgranularly fractured crystal grains on at least the sides of the piezoelectric/electrostrictive substance on which the electrodes are formed is 1% or less, and the piezoelectric/electrostrictive substance forms a curved surface near a joined section between the piezoelectric/electrostrictive substance and the ceramic substrate. This manufacturing method comprises a first step of providing a plurality of ceramic green sheets containing a piezoelectric/electrostrictive material as a major component, a second step of forming opening sections having an almost right-angled quadrilateral shape, in which at least two corners are curved, at specific positions of a plurality of the ceramic green sheets, a third step of stacking a plurality of the ceramic green sheets in which the opening sections are formed to obtain a ceramic green laminate having holes, a fourth step of integrally sintering the ceramic green laminate to obtain a ceramic laminate having holes, a fifth step of forming electrodes at least on side walls which make up the holes in the ceramic laminate, a sixth step of cutting the ceramic laminate on the holes in a direction perpendicular to the arrangement of the holes and perpendicular to the openings of the holes to obtain a comb tooth-shaped ceramic laminate, and a seventh step of cutting the comb tooth of the comb tooth-shaped ceramic laminate in a direction perpendicular to the cutting surface obtained in the sixth step and perpendicularly to the arrangement of the comb tooth.
In the first method of manufacturing a matrix type piezoelectric/electrostrictive device according to the present invention, the ceramic green laminate preferably consists of at least two types of ceramic green sheets. One of the two types of ceramic green sheets is preferably a specific number of ceramic green sheets in which a plurality of opening sections almost in the shape of a right-angled quadrilateral in which two corners are curved are formed. The other of the two types of ceramic green sheets is preferably a specific number of ceramic green sheets in which a plurality of opening sections almost in the shape of a right-angled quadrilateral and a plurality of other opening sections connected with the opening sections in the shape of a right-angled quadrilateral are formed. The other opening sections are preferably connected with the opening sections almost in the shape of aright-angled quadrilateral and connected with the ends of the ceramic green sheets.
The first method of manufacturing a matrix type piezoelectric/electrostrictive device according to the present invention preferably comprises a step of cutting the ceramic laminate, thereby opening each of the other opening sections. The cutting in the seventh step is preferably performed by wire sawing. The first method preferably comprises a step of filling space between the comb tooth with fillers after the fifth step, but before the seventh step.
A second method of manufacturing a matrix type piezoelectric/electrostrictive device according to the present invention is a method of manufacturing a piezoelectric/electrostrictive device in which a plurality of piezoelectric/electrostrictive elements almost in the shape of a pillar are formed on a thick ceramic substrate, wherein each of the piezoelectric/electrostrictive elements includes a piezoelectric/electrostrictive substance and at least a pair of electrodes, the percentage of transgranularly fractured crystal grains on at least the sides of the piezoelectric/electrostrictive substance on which the electrodes are formed is 10% or less, and the piezoelectric/electrostrictive substance forms a curved surface near a joined section between the piezoelectric/electrostrictive substance and the ceramic substrate. This manufacturing method comprises a step A of providing a ceramic green formed product containing a piezoelectric/electrostrictive material as a major component, a step B of sintering a ceramic precursor including at least the ceramic green formed product to obtain an integral ceramic sintered product, a step C of forming a plurality of first slits in the ceramic sintered product by a machining method utilizing loose abrasives as processing media, a step D of forming the electrodes on the sides of the first slits, and a step E of forming a plurality of second slits which intersect the first slits.
In the second method of manufacturing a matrix type piezoelectric/electrostrictive device according to the present invention, the ceramic green formed product is preferably formed by stacking a plurality of ceramic green sheets. The ceramic precursor is preferably formed of at least a ceramic green substrate having through holes or via holes and the ceramic green formed product. The second method preferably comprises a step of filling the first slits with fillers after the step C, but before the step E.
In the second method of manufacturing a matrix type piezoelectric/electrostrictive device according to the present invention, it is preferable to use a wire sawing method as the machining method. In the case of using the wire saw processing method, it is preferable to form the first slits and/or the second slits by performing first cutting which includes processing the ceramic sintered product in the direction of the thickness of the ceramic sintered product to obtain first cut grooves, second cutting which includes processing the ceramic sintered product in the direction of the thickness at a specific distance from the first cutting position to obtain second cut grooves, and third cutting which includes cutting the ceramic sintered product from the inside of the second cut grooves toward the inside of the first cut grooves, thereby removing regions between the first cut grooves and the second cut grooves. In this case, it is preferable to fill the first cut grooves with fillers after the first cutting, but before the second cutting.